1. Field of the Invention
The present invention relates to a new class of surface modified organic pigments, a process for their preparation and non-aqueous inks including at least one such surface modified pigment.
2. Description of the Related Art
In industrial ink jet systems, there is a constant demand for increased printing speeds in combination with high image quality. This requires an excellent dispersion quality and stability of the inks in order to guarantee continuous printing without production loss due to nozzle or engine failures. The new print heads designed for increasing printing speed only operate with very low viscous inkjet inks which generally exhibit inferior shelf life stability. In addition, the reduction of pigment particle size for image quality also tends to result in more critical shelf life stability.
Polymeric dispersants are substances for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium. They typically contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have polymer chains compatible with the dispersion medium, thus stabilizing the pigment particles in the dispersion medium.
In aqueous pigment dispersions, the polymeric dispersants generally contain hydrophobic anchor groups exhibiting a high affinity for the pigment surface and hydrophilic polymer chains for stabilizing the pigments in the aqueous dispersion medium. Thermally stable non-aqueous dispersions with submicron particles are much more difficult to prepare, especially for pigments having a non-polar surface.
Several methods to improve the dispersibility of organic pigments are disclosed in The Chemistry of Inkjet Inks. Edited by MAGDASSI, Shlomo. Singapore: World Scientific Publishing Co., 2010. p. 111-119.
Often commercially available polymeric dispersants are used as general purpose dispersants for all pigment types. However, it is known in the art that several pigments cannot be milled down to the required particle size for high end ink jet applications using only general purpose dispersants. In a first approach to optimize the dispersion quality of these pigments, dedicated dispersants have been designed, containing structural moieties, having a structural similarity with the pigment to be dispersed. Such dispersant designs have been disclosed in WO 2007/089859 (CABOT), WO 2007/006634 (AGFA), WO 2007/006635 (AGFA), WO 2007/006636 (AGFA), WO 2007/006637 (AGFA), WO 2007/006638 (AGFA) and WO 2007/006639 (AGFA). However this approach, though very effective, requires the design of a dedicated dispersant for each class of pigments, leading to high costs making this approach less attractive from an economical and industrial point of view.
In a second approach, so called dispersion synergists have been developed to improve the dispersibility of specific pigments in combination with general purpose dispersants. Dispersion synergists are especially useful to disperse pigments with a hydrophobic surface in a non-aqueous dispersion medium by modifying the pigment surface to a hydrophilic surface. Several dispersion synergists have been commercialized, such as Solsperse™ 5000 from Lubrizol. WO 2007/060254 (AGFA) discloses quinacridone derivatives having carboxylic acid groups for modifying the surface of quinacridone pigments. Other dispersion synergists are disclosed in WO 2007/060255 (AGFA), WO 2007/060259 (AGFA), WO 2007/060264 (AGFA), WO 2007/060265 (AGFA) and WO 2007/060265 (AGFA). As dispersion synergists generally show a certain molecular similarity with the pigments to be dispersed, this approach also requires the development of a specific dispersion synergist for each pigment class, leading again to a high additional cost and an economically less attractive option.
In a third approach, the pigment surface is modified by direct chemical reaction introducing polar or acid groups on the pigment surface, which should enhance the interaction with the dispersants used in the milling process.
Diazonium technology has been successfully used both on carbon black and different organic pigments to introduce functional groups as disclosed in U.S. Pat. No. 5,851,280 (CABOT) and U.S. Pat. No. 5,837,045 (CABOT). However, this technology requires an aqueous environment, making it difficult to extend the technology to solvent based and UV based inks.
Surface sulfonation has been disclosed in U.S. Pat. No. 3,528,840 (HUBER), US 2003134938 (DAINICHISEIKA COLOR) and EP 2316886 A (AGFA). Sulfonation leads to highly acidic pigment surfaces, which are incompatible with cationic radiation curable formulations or vinyl ether containing free radical radiation curable formulations, unless specific precautions are taken. Vinyl ethers, such as vinylether acrylates disclosed by EP 997508 A (AGFA), are of special interest for preparing low viscous radiation curable ink jet inks suitable for the new print heads designed for increasing printing speed.
Therefore, it would be advantageous to design carboxylation technology for surface modification of organic pigments, making the modified pigments compatible with vinyl ether containing radiation curable formulations by design.
Surface carboxylation through hydrolysis has been disclosed in WO 2008/034472 (AGFA) and WO 2004/094534 (CABOT). However, this technology is restricted to organic pigments comprising hydrolysable groups in their structure. Several industrially interesting pigment classes cannot be surface modified using this technology.
U.S. Pat. No. 6,264,733 (CIBA) discloses surface modification with a formaldehyde precursor in combination with aromatic carboxylic acids in a strong acidic medium. Strong acids are not easy to remove from pigments filter cakes by washing. Traces of strong acid catalysts render the pigment incompatible with vinyl ether containing radiation curable formulations.
U.S. Pat. No. 6,831,194 (COLUMBIAN CHEMICALS) uses cyclic anhydrides in combination with Lewis acids to introduce gamma keto carboxylic acids on the pigment surface. Also Lewis acids have to be removed to the last trace to render the pigment compatible with vinyl ether containing radiation curable formulations.
Hence there is still a need for carboxylation technology, applicable on a broad range of pigments, without the need for significant amounts of strong acids or Lewis acids as catalysts.